Quality control or testing of a lens system to determine whether it is capable of functioning adequately within its design parameters is of great importance, particularly in the case of high performance lens systems, and is becoming increasingly important with the development of lens systems for highly sophisticated applications. Lens systems have been developed which require very high contrast in terms of lines/mm (modulation transfer function (MTF)), and minimum phase shift (phase transfer function (PTF)). Both MTF and PTF define the optical transfer function (OTF) of the lens. In many instances, particularly in the case of lens systems with high refractive index elements and low F number, it is also important to determine the polarization characteristics of the lens.
The prior art has developed a limited number of practical systems for testing the MTF of a lens, which employ a blackbody or monochromatic incoherent light source and a physical object or target device, such as a grating having parallel lines of fixed spatial frequency employed in conjunction with a narrow slit. By rotating the grating relative to the spatiallyfixed object slit, spatial frequency can be changed within limits. To extend the range of test spatial frequencies, the grating or relay optics must be changed. A second slit positioned in the image plane at right angles to the object slit provides a small rectangular scanning aperture for the transmitted modulated light. It should be noted that this cumbersome means of generating different spatial frequencies also generates a continuously changing pattern shape of the target with related distortions. Except for the most complex and expensive system presently available, PTF cannot be effectively measured and, in the exception, can be measured only within a minimum error of 5 percent. For a detailed description of the present state-of-the-art in OTF testing, see D. F. Horne, Optical Production Technology, Crane, Russak & Co., New York, 1972, pp. 363-382.
Other disadvantages of the prior art systems include the following:
They operate practically only in the visible optical wavelength range -- a serious problem for the rapidly developing and important art in infrared and ultra violet optics.
The normalized or absolute amplitude level, which is obtainable only at zero cycles per second of spatial frequency, is difficult to obtain with a desired degree of accuracy. Normalization error as low as 1 percent appears to be attainable only with the most highly complex and expensive monitoring systems presently available. Failure to normalize the MTF curve accurately at zero results in considerable scaling error of the curve at all points.
The characteristic generation of a one-dimensional line pattern or array of dark and light zones, e.g., along the y-axis, tends to introduce distortions in comparision with the two-dimensional array in the x and y directions produced by the test lens in actual use.
The present prior art systems also do not determine the polarization characteristics of the lens.
The present invention employs a laser interferometer system capable of producing, by convergence of two coherent-radiation beams of equal size and intensity and of slightly different frequency f.sub.o and f.sub.o.sub.', the .DELTA.f of which is in the radio frequency range, a laterally-moving-fringe pattern. The generation of laser interferometer fringe patterns and their use in sensing particle velocity and size in a moving fluid medium by separating the AC and DC signal components of the radiation scattered by the particles and determining contrast from the AC/DC ratios, are disclosed in detail in the following articles: (1) W. M. Farmer, "Measurement of Particle Size, Number Density and Velocity Using a Laser Interferometer," Applied Optics, Vol. 11, No. 11, Nov. 1972, pp. 2603-2612; (2) W. M. Farmer et al, "Two-Component, Self-Aligning Laser Vector Velocimeter," Applied Optics, Vol. 12, No. 11, Nov. 1973, pp. 2636-2640; and (3) W. M. Farmer, "Observations of Large Particles With a Laser Interferometer," Applied Optics, Vol. 13, No. 3, March 1974, pp. 610-622.
The laser interferometer systems, as disclosed by the prior art and particularly the laterally-moving-fringe pattern system described by Farmer (3) supra, is employed as a basic element of the present apparatus and process with additions and modifications essential to accomplish the purposes of the invention. An appropriately positioned beam splitter is positioned downstream of the initial laser beam splitter, to divide the original f.sub.o and f.sub.o.sub.' beams into two sets of f.sub.o and f.sub.o.sub.' beams of the same size and intensity, with one set functioning as the test channel and the other as the reference channel. Each beam set is converged to form first and second spatially separated, laterally moving-fringe patterns. The first functions in the object plane of the test lens as the test target and the second as the reference. The first test pattern is formed into a real image by an appropriate, radiation-scattering, imaging plate positioned across the fringe pattern of the object plane. The target light pattern formed on the real-imaging plate is then reimaged by the test lens system and analyzed through a first slit of appropriate size positioned in the image plane of the lens system being tested. The second reference fringe pattern is simultaneously analyzed through a second slit. The scattered radiation transmitted through the test and reference slits are each collected and separated into AC and DC signal components. The AC/DC ratios, which are a measure of contrast (MTF), are computed and compared with the aid of associated conventional electronics. The phase of the test and reference AC signal components are also electronically compared and any phase shift (PTF) is simultaneously determined.
Spatial period of the test fring-pattern target (.lambda..sub.s) and reference (.lambda.".sub.s) can be continuously or incrementally varied from infinity through a finite range determined by the design characteristics of the OTF measuring system and the design requirements of the test lens and its diffraction limits, simply by varying the angle of convergence .theta. of the f.sub.o and f.sub.o.sub.' beams from zero through a predetermined degree of arc. The target pattern is always the same in shape except for the spatial frequency of the fringes, since the fringes are parallel to each other and to the plane defined by the x and z axes of the fringe planes. Although splitting of the initial laser beam into test and reference channels is the preferred embodiment, other means of providing the requisite reference can also be used.
The test lens can be rotated around its optical axis and turned around the nodal point in a fashion similar to that employed in present state-of-the-art techniques.
The advantages of the system of the present invention include but are not limited to the following:
The system can be employed to test optical systems in a .lambda..sub.o range from ultra violet or shorter through far infrared or longer by employing a laser source producing the required wavelength, optics, such as the Bragg cell, the converging lenses and real-image forming plate, which are transparent to the particular wavelength and detectors sensitive to the particular .lambda..sub.o, all of which, with the possible exception of the real-image forming plate which at present would probably require custom fabrication, are conventionally available.
The test fringe target eliminates the cumbersome physical target means and means for generating different spatial frequencies presently employed and provides a more uniformly patterned and, therefore, less distorted target as spatial frequency is varied to determine OTF.
PTF can be determined within about 1 percent or less, a degree of accuracy which is not presently available. The use of a polarized radiation source makes it possible to test polarization characteristics of the lens system.
The fringe zone comprises a two-dimensional pattern in the plane of the x- and y- axis, which provides more accurate test readings in terms of actual performance of the test lens during use.
The normalized zero MT reading is obtained by a convergent angle equal to zero so that the f.sub.o and f.sub.o.sub.' beams are coincident, thereby producing a fringe pattern having a fringe period of infinity. The resulting pattern is a single spot of radiation of uniform illumination at any given instant in time. The percentage of error is generally one percent or less, whereas such accuracy can only be obtained with the most sophisticated prior art equipment presently available.
Use of a moving fringe pattern as the target means provides both improved accuracy of the test pattern and an extended range of spatial frequency in terms of lines/mm by eliminating a mechanically produced target and by the dependence of the test pattern solely on the wavelength of the radiation source and the converging angle, so long as the converging beams are diffraction-limited by use of high quality optics.
System imperfections generally can be minimized or compensated because of the fewer optical components required, as compared with the present state-of-the-art systems and by the fact that, except for the real-imaging screen, all of the optical components can be paired in the test and reference channels and can be, therefore, compensated.
The system of the invention can also test the lens system at different levels of contrast of the object merely by changing the RF driver power of the initial laser beam-splitter, such as a Bragg cell. Measurement at different contrast levels is apparently not feasible with present prior art test equipment. The present invention, in addition to its versatility and its capability for accurately measuring both the MTF and PTF comprising the OTF of a lens system, as well as its polarization and chromatic aberration characteristics, has the advantage of relative simplicity, reduced number of optical components and reduced cost.
None of the available art, to the extent known, discloses the present invention. It will be understood that the terms "lens" or "lens system" (namely, the imaging system) as employed in this specification and claims encompasses a single lens, a plurality of adjacent lenses, or a lens or plurality of adjacent lenses associated with conventional electronic imaging tubes, such as image intensifiers, vidicon tubes, IR imaging tubes, image dissectors, and the like.
It will also be understood that the wavelengths .lambda..sub.o and .lambda..sub.o.sub.' corresponding to the split f.sub.o and f.sub.o.sub.' beams, are so minutely different in size that comparison of wavelength size to such elements as image slit size or scattering-center size refers to both .lambda..sub.o and .lambda..sub.o.sub.' and in this context the terms .lambda..sub.o and .lambda..sub.o.sub.' are interchangeable.